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Lupinus lepidus, a prostrate, evergreen perennial and Lupinus latifolius, a robust, deciduous perennial were prominent survivors and among the first colonizers of early successional habitats following the 1980 eruption of Mount St. Helens. In this study, interspecific and age-specific variation in the water relations and photosynthetic properties of these lupines were assessed to determine their physiological capacity to colonize these volcanic habitats. Comparative water-relations studies showed that seasonal and diurnal trends in plant water balance were similar between species and age classes. Stomatal conductance and xylem pressure potential of seedlings and adults were closely coupled with soil water potential and leaf-to-air vapor-pressure gradients. Relatively high osmotic potentials were linked with high leaf water contents and cell-wall elasticity. Collectively, these water-relations properties are indicative of mesophytic, herbaceous species with leaf tissues sensitive to soil and atmospheric water deficits. The photosynthetic capacity of L. lepidus was higher over a wider range of environmental conditions than that of L. latifolius. L. lepidus had a higher light saturation point and was able to acclimate photosynthetically to higher temperatures, relative to L. latifolius. The photosynthetic activity of L. lepidus seedlings was significantly greater at higher light and temperature regimes compared to seedlings of L. latifolius. Differing photosynthetic responses to temperature and light help explain the contrasting demographic patterns of these lupines and the relative dominance of L. lepidus in early successional habitats on Mount St. Helens. The physiological capacity to respond to extreme temperatures and periodic water deficits also appears to be an important determinant of the natural distribution patterns of these two species in the Pacific Northwest: L. lepidus occupies windy, exposed alpine habitats, whereas L. latifolius is common within subalpine meadows and coniferous treeline communities. |
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